Compat.hs

{-# LANGUAGE ExistentialQuantification #-}
{-# LANGUAGE UndecidableInstances #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE ExplicitNamespaces #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE Rank2Types #-}
{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE CPP #-}

module GHC.TypeLits.Compat
    ( -- * Kinds
      Nat, Symbol
      -- * Linking type and value level
    , KnownNat, natVal
    , KnownSymbol, symbolVal
    , type (<=)
    , type (<=?)
    , type (+)
    , type (*)
    , type (^)
    ) where

import GHC.TypeLits

#if __GLASGOW_HASKELL__ < 707
data Proxy k = Proxy

class SingI n => KnownNat (n :: Nat) where
    natSing :: Sing n

instance SingRep n Integer => KnownNat n where
    natSing = sing

natVal :: forall n proxy. KnownNat n => proxy n -> Integer
natVal _ = fromSing (natSing :: Sing n)

class SingI n => KnownSymbol (n :: Symbol) where
    symbolSing :: Sing n

instance SingRep n String => KnownSymbol n where
    symbolSing = sing

symbolVal :: forall n proxy. KnownSymbol n => proxy n -> String
symbolVal _ = fromSing (symbolSing :: Sing n)
#endif